Printable hydrogels desired in bioengineering have extremely high demands on biocompatibility and mechanic strength, which can hardly be achieved in conventional hydrogels made with biopolymers. Here, we show that on employment of the strategy of coordination-triggered hierarchical self-assembly of naturally occurring small-molecule folic acid, supramolecular hydrogels with robust mechanical elastic modulus comparable to synthetic double-network polymer gels can be made at concentrations below 1%. A sequence of hierarchical steps are involved in the formation of this extraordinary hydrogel: petrin rings on folate form tetramers through hydrogen bonding, tetramers stack into nanofibers by π-π stacking, and zinc ions cross-link the nanofibers into larger-scale fibrils and further cross-link the fibril network to gel water. These supramolecular qualities endow the hydrogel with shear-thinning and instant healing ability, which makes the robust gel injectable and printable into various three-dimensional structures. Owing to the excellent biocompatibility, the gel can support cells three-dimensionally and can be used as an ideal carrier for imaging agent (Gd), as well as chemodrugs. In combination with its easy formation and abundant sources, this newly discovered metallo-folate supramolecular hydrogel is promising in various bioengineering technological applications.

Polyvinyl alcohol (PVA) hydrogels have been proposed for use as promising biomaterials in biomedical and tissue engineering, and graphene oxide (GO) has been recognized as a unique two-dimensional building block for various graphene-based supramolecular architectures. In this article, we systematically studied the influence of three kinds of PVA with different molecular weights on the interaction between PVA and GO. Moreover, the effects of PVA on the gelation of GO were also investigated. The native PVA hydrogel, as well as PVA-GO hybrid hydrogels, have been thoroughly characterized by the phase behavior study and various techniques including field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA) and rheological measurements. It can be seen that with the increase of the molecular weight of PVA, the addition of GO can effectively promote the gelation of PVA which can be reflected by a decrease of the critical gel concentration (CGC) for PVA-GO hydrogels. Dye adsorption experiments indicate that the toxic dye, i.e., methylene blue (MB), was efficiently entrapped in the PVA-GO xerogels. It is also demonstrated that the gelation of PVA and GO composites can be promoted by different supramolecular interactions, including hydrogen bonding and electrostatic interaction. This work indicates that the PVA-GO composite is a good candidate for preparing "super" and "smart" hydrogels and will enable further studies on the supramolecular chemistry of PVA, graphene and its derivatives.

Caking of powder materials is undesired in various industries, and for thousands of years people are fighting against caking. Herein, the principle of caking is employed to create macroscopic plastic supramolecular films through a cold sintering process. A nanometer-sized, irregular coordinating cluster is first generated with a bulky head surfactant and multifunctional ligand, and the addition of metal ions immediately leads to amorphous white precipitates. Upon adsorbing moisture, a rearrangement of the molecules in the precipitates results in cold sintering, so that the particles in the precipitates grow into a transparent macroscopic film. The mechanical strength of the film is comparable to plastics, but allows welding and molding with finger at ambient temperature in moist environment. Mechanical tests suggest the supramolecular plastic does not fatigue even after several tens circles' remolding, indicating their superior material engineering capability. This strategy can be extended to different chemistries to fabricate films with different mechanical strength. Various functional components can be doped into the resultant films, rendering them a platform toward multifunctional materials, such as luminescent devices or sensors for pollution gases. The current strategy opens up a new vista in material science is expected.